The principles of sensory reception and motor function are being studied, with Salmonella typhimurium as the model system. This bacterial species, like many others, is motile and migrates in response to chemical gradient stimuli in its environment. Using high intensity dark field microscopy, we are examining the coordinated operation of the motor organelles (thin helical filaments called flagella) during translational movement, and the more complex events comprising the "tumble", a randomizing process essential to migrational responses.